The principal source of metallic indium is represented by complex ores containing a number of valuable components. Thus, lead-zinc ores which are main raw materials for the production of metallic indium also contain sulphur, selenium, tellurium, copper, cadmium, arsenic, thallium, gallium, germanium and other components. Indium produced from this starting material contains a substantial amount of contaminating components which may be removed by combination of several methods for purification of indium; each of said methods being effective for purification from only individual limited impurities in the metal.
Known in the art is a method for producing high-purity indium using transfer reactions (cf. FRG Pat. No. 1,229,304 Cl. 40a 61/00; 1966). This process makes it possible to produce the metal with a content of the main substance of 99.999 to 99.9999% by mass. The process contemplates creation, in the reaction vessel, of a temperature drop, preferably 100.degree. C.; molten indium within the range of higher temperatures is subjected to the influence of a gas mixture of hydrogen and water vapours. As a result, molten indium is oxidized into a lower oxide which is transferred to the reactor zone having lower temperature and reduced, therein, to the highpurity metal. This process features a low productivity and necessitates high power and labour costs.
In the prior art process for the production of high-purity indium by chlorination of metallic indium, followed by processes of rectification, zone melting and disproportionation of the purified monochloride it is possible to obtain indium with a content of the main contaminants of the order of 10.sup.-5 % by weight (cf. Bronnikov A. D., Vasilevskaja I. I., Niselson L. A., Nikolaev R. K., Smirnov V. A., Izvestija AN SSSR, Metals, 1974, No. 4, p. 54). However, the use of chlorine or chlorine-containing products complicates the process equipment, necessitates special safety precautions, while an increased hygroscopicity of indium monochloride hinders effective zone melting and rectification processes. Among other disadvantages of the process there should be mentioned its multi-stage character and increased losses of indium reaching 10%.
The process for electrolytical refining of indium (cf. U.S. Pat. No. 3,268,425; Cl. 204-105, Int.Cl. B 01 K; 1966) stipulates anodic dissolution of indium containing an increased amount of impurities. The hydrochloric-acid solution has a pH value of 0.5 to 1 and contains chlorides of indium and sodium in an amount of about 80 g/l. Graphite cathodes are uniformly positioned between the anodes; recovery of of purified indium takes place on said cathodes. During electrochemical refining of indium there occurs the exchange of the electrolyte solution with its purification on ion-exchange resins of a quaternary ammonium base. The process makes it possible to produce indium of an increased purity; however, metallic indium deposited on the cathode has an extremely developed surface thus causing an enhanced oxidation of the metal, entrainment of the electrolyte and, finally, contamination of the purified metal and lowered yield of the final product. The process does not enable a sufficiently deep purification of the metal (below 1.10.sup.-5 % by mass) from a number of limited impurities such as thallium, cadmium, zinc, lead and others.
Known in the art is a process for electrolytical refining of indium from an acidic solution using, as the cathode, metallic mercury (cf. Japanese Patent No. 427/1970; Cl. 10P23, 1970). However, the use of mercury complicates safety conditions during refining of indium, gives rise to the problem of purification of indium and amalgam saturated with impurities from mercury.
In another prior art process (cf. Reinststoffe Wissenschaft und Technik, Berlin, 1963, 91-104) use is made of amalgam electrolysis of indium, followed by vacuum melting of indium at a temperature within the range of from 550.degree. to 600.degree. C. under a residual pressure in the chamber of from 1.10.sup.-4 to 1.10.sup.-5 mm Hg over a period of from 3 to 4 hours. The resulting metallic indium contains 99.999% by mass of the main substance. This prior art process has the same disadvantages as those inherent in the above-described one.
Also known is a process for producing high-purity indium (Polish Application No. 65539; Cl. 40 c 1/06 1972), wherein indium with an increased content of impurities is dissolved in hydrochloric acid, neutralized with a solution of ammonia; afterwards, the indium-containing solution is purified by cementation of the impurities on indium sheets of a high purity, while indium is isolated from the resulting solution by electrolysis, heated in vacuum at the temperature of above 1,000.degree. C.
In the above process there take place substantial losses of the purified metal due to the multi-stage process scheme, large amounts of the solutions employed and high temperatures of vacuum treatment of the metal. The multi-stage character of the process exerts a detrimental effect on purity of the resulting final product.